It has been suggested to indicate, or measure contact pressure distribution by employing electronic or mechanical pressure transducers. However, many of these transducers are quite large and bulky, and, even if miniaturized, it is difficult if not impossible to adapt for use in a limited space, such as at the interface between machine elements, (e.g. paper and metal rolling mills, pressing machines, ball or roller bearings, etc.).
Furthermore the simple presence of such transducers is disturbing the contact that has to be evaluated. An other limiting characteristic of the above situation is lack of access; so, light can not be directed to the gauge location, optical data can not be observed, electrical and mechanical connections can not be mounted.
Two methods using materials exibiting mechanical birefringence properties (photoelastic or photoplastic properties) have been suggested so far for measuring contact pressure distribution; both of them are using sheets of such materials and are converting pressures perpendicular to them into "in-plane" stresses or strains:
(a) One approach is disclosed in U.S. Pat. No. 3,966,326 issued to Brull and Arcan. In accordance with the teachings of this patent, the pressure transmitting member (a foot or a shoe e.g.) are transfering the load to a plurality of point-contact projections that are discretizing and transmitting the pressure stress to a photoelastic sheet.
The characteristics of this approach are:
using an elastic (photoelastic) sheet, a real-time response is obtained in a field of circular polarized light, PA1 being induced by point-contact (axial-symmetric) projections, this response is exibiting a pressure pattern of circular isochromatics due to the axial symmetric deformation, PA1 the contact intensity is given by the diameter of the first order isochromatic surrounding the contact point and calibration is possible in terms of local force, PA1 at the contact point, the local values of the "in-plane" principal strains .epsilon..sub.1 and .epsilon..sub.2 are equal and so the shear strain .gamma. (equal to their difference) is zero, which means that the local birefringence is zero. PA1 using a photoplastic material, a memorized response is obtained, PA1 the plastic material is locally extruded under the teeth loading, and the contact intensity is given by the peak value of birefringence, right at the contact points or regions, PA1 at these contact points (because teeth are not axially symmetric), the two local principal strains .epsilon..sub.1 and .epsilon..sub.2 (in the plane of the sheet) are of different values and their difference .gamma. is providing the peak value of birefringence to be calibrated versus the local strain intensity (perpendicular to the sheet). PA1 the load transmitting member is inducing in the plane of the sheet uniaxial deformation .epsilon..sub.1. PA1 it may be calibrated in terms of strain, stress, force, force per unit of length, PA1 its geometry and flexibility makes it adaptable to many environments, PA1 it is able to directly record the maximal values of stress or force unit
(b) An other approach is disclosed in U.S. Pat. No. 4,324,547 issued to Arcan et al to be applied in the dentistry field to evaluate bite characteristics. This method is using photoplastic material sheets exibiting important permanent birefringence (or memorized birefringence). The above mentioned property is needed to record the bite and the localized peak strains imparted to the sheet by the teeth in order to quantify, afterwards (in a field of polarized light) the bite characteristics. There is no real-time alternative analysis because no access for light and observation is possible during occlusion.
Hence the characteristics of this approach are:
In the engineering field it is often desirable to quantitatively examine the contact pressure conditions, or distribution at the interface between machine elements, such as paper and metal rolling mills, pressing machines, meshing gear teeth, and the like. In fact, in the fields of Tribology (i.e. science of contact friction) and Interface Mechanics (static and dynamic loading--including explosive) the quantitative measurement of interface contact pressure distribution is very desirable. However, in many environments access to the interface is virtually impossible and, in most of these environments, the inclusion of a transducer between the surfaces to be investigated will disturb the normal contact pattern; thereby introducing significant error into the measurement. Clearly, the environment provided at the interface between machine elements does not permit the use of a real-time method for observing localized point-like projections effect of the general type disclosed in the '326 patent issued to Brull et al.
On the other hand such an environment is very different from the indenting one provided by the dental arches where the shapes of teeth are so different and difficult to define that a calibration in terms of stress or force is not possible.